ZD1937,pdf datasheet (Step-Up

The ZD1937 is a constant frequency, current mode PWM,
inductor based step-up (boost) converter especially designed
to drive multiple white LED’s in series. The LED current is
regulated in this converter design so that all LED’s have the
same current and therefore have inherently matched light
output. The converter design operates at high frequency
(1.2MHz) operation so that a small, low profi le inductor can
be used as well a small value output integrating capacitor.
The ZD1937 has built-in over-voltage protection (OVP) with
internal thermal shutdown ability to prevent device from overvoltage
or an open circuit condition at the voltage output.
The ZD1937 is available in either a space saving SC70, a
SOT25, or ThinSOT25 package for very small board space
applications.